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The Milky Way's Dark Shell

The galaxy's main ingredient is also its most inscrutable: a cloud of dark matter several times more massive than the visible stars and gas. Astronomers don't know what kind of matter makes up this cloud, but now they know at least its shape. Observations reported today at a meeting of the Royal Astronomical Society in Southampton, England, show that the Milky Way is wrapped in a spherical halo of dark matter.

Astronomers have known of the effects of dark matter for decades. Galaxies like the Milky Way are surrounded by swarms of ancient stars, called globular clusters, which orbit the center of the galaxy faster than the gravity of visible matter alone can explain. Some covert source of gravity--the dark matter--has to be at work, but because it gives off no light, researchers must devise clever techniques to probe it indirectly.

Robert Olling and Michael Merrifield, astronomers at Southampton University in the U.K., thought the layer of radio-emitting hydrogen gas that pervades the disk of the Milky Way might betray the shape of the dark-matter halo. "You map the hydrogen in the sky, and you can see a disk that traces out the Milky Way," says Olling. A computer model showed that the gravity of the dark matter would influence the shape of the hydrogen disk, with a flattened, pancake-shaped halo of dark matter pulling hydrogen gas into an even, thin disk. But when the team mapped radio waves from the hydrogen layer, they found a shape that seemed to point to a spherical halo of dark matter: The gas layer thickened toward its edges where a lower density of dark matter would have less tug.

Other astronomers welcome the finding, which confirms their long-standing suspicion that the dark matter mirrors the spherical distribution of globular clusters. "It's the first time we have a quantitative indication," says Roger Ferlet of the Institut d'Astrophysique de Paris.